1. Field of the Invention
The present invention relates to the control of macroscopic liquid crystal alignment in a liquid crystal layer, and to an optical device comprising a liquid crystal layer having its macroscopic liquid crystal alignment controlled in this way.
2. Description of the Related Art
FIG. 1 of the accompanying drawings illustrates the co-ordinate system of a LC director (average molecular orientation) in space. The angle θ is the zenithal (out of plane) angle while φ is the azimuthal (in-plane or planar) angle. The angle θ is also known as the tilt angle while φ is also known as the twist angle. The LC layer (or substrate) is in the x-y plane.
Patterning of liquid crystal alignment on a gross scale is known. In this context, gross patterning refers to alignment regions that have a lateral extension larger than the thickness of the liquid crystal layer. The resulting liquid crystal layer has at least two domains that are optically distinct with the patterning in one region having negligible effect on the patterning of a neighbouring region.
Patterning of alignment on a fine scale (˜1 μm) in liquid crystal devices has also been disclosed previously. “Surface alignment bistability of nematic liquid crystals by orientationally frustrated surface patterns” Kim et al, Applied Physics Letters, 78, pp3055 (2001) describes azimuthal micropatterning in two directions and demonstrates bi-stable bulk director orientations. “Tristable nematic liquid crystal device using micropatterned surface alignment” Kim et al, Nature 429, pp159 (2002) describes azimuthal micropatterning in three directions and demonstrates tri-stable bulk director orientations. Kim et al do not disclose spatially varying the proportions of the patterned alignment regions or mention possible devices that could result from such patterning. The azimuthal patterning in both these papers is achieved using an atomic force microscope (AFM) to rub the substrate and achieve domains that have a width of 4 μm (down to 0.5 μm). However, it takes between 2 and 10 hours to rub an area the size of a single pixel (90 μm×90 μm) using an atomic force microscope (AFM). Consequently, AFM rubbing is not considered to be a production-viable technique
“Alignment of nematic liquid crystals by inhomogeneous surfaces” Ong et al, Journal of Applied Physics, 57, pp186 (1985) describes the use of a surface composed of patches that favour homeotropic alignment that are surrounded in a matrix favouring planar alignment. The resulting zenithally patterned surface produces a bulk pretilt that is between planar and homeotropic. The pretilt is a function of the area ratio between the homeotropic and planar alignment regions. However, this zenithal patterning method relies on SiO and metal film deposition, and SiO deposition is not considered a production-compatible technique. The fabrication method described by Ong et al could not be used to spatially vary the proportions of the homeotropic and planar alignment directions. Therefore the average bulk zenithal angle will be the same everywhere (i.e. no spatial variation of zenithal retardation is possible). Ong et al do not disclose spatially varying the proportions of the patterned alignment regions or mention possible devices that could result from such patterning.
Zenithally bistable displays (ZBD) have been disclosed in several patents, for example in WO 97/14990. All ZBD devices have a locally uniform surface pretilt on a grating structure The resulting bulk optical properties of the ZBD are a consequence of the topology of the undulating surface.
The techniques mentioned above are considered either to rely on commercially incompatible production methods or to have insufficient patterning flexibility to enable a number of novel optical elements to be made, or both.